This invention relates to rotor cooling systems for dynamoelectric machines and in particular to slot wedges which may be employed to facilitate the flow of cooling fluid.
Dynamoelectric machines, particularly those employed in power generation, generally comprise a cylindrical slotted stator core through which conductive stator windings are disposed and a cylindrical, slotted rotor core through which conductive rotor winding bars are disposed. The rotor windings are conventionally connected to energizing and excitation circuitry through the use of slip rings and carbon brushes or other electromagnetic coupling means. The rotor provides a substantially constant magnitude, rotating field which is radially directed with respect to the generally cylindrical rotor assembly. Because of the rotary motion, the field lines associated with the magnetic flux are made to cross stationary bars of insulated conductive material in the stator assembly so as to induce an electrical voltage in the stator winding bars. Direct electrical connections to the bars in the stator winding provide three-phase electrical power such as that generated by most electric utilities. Various rotor and stator constructions are conventionally known in the art.
Because of the generally increasing costs associated with certain fossil fuels, the energy efficiency of power-generating equipment has become a highly significant performance parameter. A large amount of energy loss in a conventional dynamoelectric generator occurs in the rotor assembly and its associated electrical windings. Because of the relatively large amounts of electrical power which are supplied through the rotating field windings, undesirable thermal energy is produced in the rotor. (Similarly, resistive heating losses also occur in the stator windings. However, the stator windings may be relatively easily cooled, for example, by water. Thus, because of the fixed position of the stator, its cooling is relatively easily accomplished.) The rotor, however, requires sophisticated cooling designs. It is to be noted that if insufficient cooling occurs for the rotor, the resultant temperature increase raises the resistivity of copper field windings, thereby further increasing the resistive heating losses. In short, it is seen that rotor cooling is essential and that improvements in rotor cooling are significant since a large source of energy loss presently occurs in dynamoelectric machine rotors.
A clever means for cooling the field windings on the rotor is illustrated in U.S. Pat. No. 2,986,664 issued May 30, 1961 to David M. Willyoung et al. This patent describes a diagonal flow cooling system for a set of rotor windings in which two sets of diagonally directed passages are disposed through the rotor windings. In this patent, the field winding bars in any given slot have a sequence of diagonally directed passages extending in one downward direction along one side of the winding bars and a second set of diagonally directed passages directed in a diagonally opposite direction along the other side of the winding bars. This diagonal flow cooling system is particularly illustrated in FIG. 4 of the above-mentioned Willyoung et al patent, which patent is hereby incorporated herein by reference as background material for the present invention. Cooling gas is introduced into these diagonal passages through a system of scoops and ducts located in the wedges which are used to retain the winding bars in the rotor core slots. Because of the relatively large diameters of the rotors and because of their rotational speed, the retention function of the wedges is important because of the high centrifugal forces developed during operation.
It is to be particularly noted that, with respect to the above-mentioned patent to Willyoung et al, flow division for separating the two diagonal flow directions does not occur in the wedge itself. This function is performed by a flow-dividing vane disposed across the holes located in a creepage block.
Rotor cooling and the importance of the gap pickup wedges is recognized in U.S. Pat. No. 3,440,462 issued Apr. 22, 1969 to David M. Willyoung (as sole inventor). This patent recognizes the difficulty of machining curved passages in the rotor wedges and accordingly proposes a wedge design in which the passages are located at the end of rotor segments so that they may be easily machined. This rotor wedge design facilitates the transition from a tangential flow to a strictly downward (or inward) directed radial flow. However, there is no provision for facilitating the transition of the flow into components having oppositely directed longitudinal velocity components. Such a flow dividing requirement appears to be inconsistent with the end-machining of the gap pickup passages. Furthermore, at least a portion of the flow splitting function resides in the dividing vanes which straddle the holes in the creepage block. Nonetheless, the patent to Willyoung appreciates the fact that the tendency for flow separation is increased if there are discontinuities or sharp changes in the direction of the flow. Furthermore, no provision is made in the patent to Willyoung to facilitate a transition from a tangential flow to a flow directed radially inward and longitudinally outward. Since the vanes across the holes in the creepage block contribute to a pressure drop across the creepage block, they tend to reduce the rotor cooling capacity and to lower efficiency more than is necessary in light of the present invention.
The importance of rotor cooling is also recognized in U.S. Pat. No. 3,995,180 issued Nov. 30, 1976 to Walter B. Giles. This patent is solely directed to increasing the flow of cooling gases through the exit portions of the diagonal flow rotor cooling system. However, no means is provided for transitioning the velocity of the gas flow from a radial and longitudinal direction to a tangential exit direction. Moreover, flow separating means are still provided in the creepage block itself.
It is also recognized by those skilled in the generator arts that reducing the size of the creepage block means that greater space may be provided in the rotor slot for the conductive rotor windings. Thus, since a larger cross-sectional area may be allocated to the winding conductors, the resistance of the field windings may be significantly reduced. This reduction naturally reduces the resistive heating losses in the rotor and can further increase overall generator efficiency.